1. Field of the Invention
The invention relates to an image-retransfer sheet for dry-processing type image-transferring materials wherein images such as letters, symbols, and figures are transferred with pressure to form such letters, symbols, and figures on an image-receiving material. More particularly, the invention relates to an image-retransfer sheet which is adapted for use in dry-processing type image-transferring materials having images formed by printing and heat-sensitive image transferring by use of heat-sensitive image transferring printing devices such as printers, typewriters, and word processors.
2. Description of Related Art
Hitherto, a technique relating to a base sheet for dry-processing type image-transferring materials which have an image printed by a thermally image-transferring process is set out, for example, in U.S. Pat. No. 4,870,427.
In this technique, the base sheet used is one which comprises a substrate, which has a contact angle with water of at least 95.degree. and is made of a sheet consisting of a film of polyethylene, polypropylene or a fluorine resin, or a paper sheet, a metallic foil or a plastic film such as a polyethylene terephthalate (PET) film, and a layer of a surface treating agent made of a silicone resin and formed on one side of the substrate. By the formation of the layer of the surface treating agent, ink images fixedly secured through thermal transferring are reliably released from the base sheet by pressure-sensitive transfer to ensure retransfer of the images onto an image-receiving material.
In order to thermally print an image onto a base sheet having an angle of contact with water of not smaller than 95.degree. (preferably not smaller than 105.degree.) or a sheet having good release properties (or having poor wettability), it is necessary to liquefy an ink and to reduce the surface tension of the ink to wet the sheet therewith. It is also necessary to increase the adhesion force between the sheet and the ink to an extent greater than the cohesive force of the ink and the adhesion force between the ink and an ink-releasing sheet such as a polyethylene terephthalate (PET) film. To this end, the ink temperature has to be increased when the image is thermally printed, thus requiring high printing energy to be applied to the thermally image-transferring device. This is disadvantageous from the standpoint of thermally image-transferring devices with respect to the durability of their printing head and the load on the power supply.
Images thermally printed on or transferred to a base sheet having poor wettability are readily retransferred by the application of slight pressure owing to a weak adhesion to the base sheet. This will undesirably permit unnecessary portions of the image to be retransferred, thereby causing stains on the image-receiving material. In addition, there arises another problem that when slightly touched during handling, the retransferred image is readily removed.
Moreover, since the base sheet has an extremely small static friction coefficient, the sheet is apt to move and is not readily fixed in position when the thermally transferred ink image is retransferred onto an image-receiving material. This results in the image not being retransferred in position or being distorted. Thus, a complete image cannot be obtained.
In order to prevent undesirable movement of the sheet at the time of retransfer of an image by pressing contact as set out hereinabove, Japanese Laid Open Patent Publication No. 63-246298 proposes a technique of forming a self-adhesive layer on a base sheet separately from a transfer pattern.
However, the partial formation of the self-adhesive layer in position requires a specific manner of formation along with an additional device therefor, thus resulting in great costs. In addition, the self-adhesive layer has to be covered with a release paper before use, which, in turn, requires additional devices and costs. Moreover, with a base sheet which has a releasable surface treating agent layer made of a silicone resin, it is necessary to coat the base sheet with two layers including the layer of a surface treating agent and the self-adhesive layer. Regardless of the coating order of the two layers, there is a fairly large possibility that the later coating is, more or less, affected. More particularly, when a silicone resin is first coated, the subsequent coating of a self-adhesive composition is repelled. On the other hand, when the self-adhesive composition is first coated, the silicone resin is unlikely to coat owing to the stickiness of the self-adhesive. If a release sheet is attached to the self-adhesive coating in order to prevent stickiness, the subsequent coating is not easily performed due to the effect of the thickness of the release sheet. In the event that two coatings can be formed without any problem, the self-adhesive layer is applied only partly. This means that all the circumferential portions of an image are not fixed and the sheet is still likely to move, making it very difficult to form a complete image.
In order to solve the above problems, in U.S. Pat. No. 4,973,509, surface treating agents to which materials having elastic or elongation properties are added have been proposed.
The surface treating agent set out in the U.S. patent comprises a silicone for controlling the wettability of a base sheet and an ethylene-vinyl acetate copolymer or polyethylene for imparting elongation properties to the sheet. The agent exhibits an elongation of at least 100%.
The surface treating agent contains, as the silicone for controlling the wettability of the sheet, either a silicone resin or a silicone oil, or both. For forming the layer of the surface treating agent on a base sheet, a coating composition dissolving a silicone resin therein is applied onto the base sheet and dried. In order that, after removal of the solvent from the coating composition by drying, the surface treating agent layer comprising the silicone resin is left on the base sheet, the molecules of the silicone resin in the composition have to be cured through crosslinkage. This requires an elevated temperature. Accordingly, for complete drying and curing within a short time, the coating composition having the silicone resin dissolved therein has to be coated in reduced amounts. This eventually leads to a lowering of the coating speed with very high production costs.
The curing of the silicone resin requires the addition of a curing catalyst (crosslinking agent) to the coating composition. The curing of the silicone resin gradually proceeds owing to the addition of the catalyst and the curing of the coating composition itself gradually proceeds. This entails a short pot life for the coating composition.
On the other hand, with silicone oils being added, a coating composition containing such a silicone oil is applied onto a base sheet and dried to remove the solvent from the solution, thereby permitting the layer of the surface treating agent layer to be left on the base sheet. Problems in this case are not involved immediately after formation of the coating layer. As time passes, after the formation of the layer, however, the silicone oil in the surface treating agent layer gradually migrates toward the surface of the coating layer. This causes print failure or image-retransfer failure.